1. Field of the Invention
This invention relates to liquid crystal display device capable of displaying in a transmissive, reflection or transflection mode.
2. Background Information
Liquid crystal display (hereinafter referred to as “LCD”) devices which have been currently used are those of a TFT (Thin Film Transistor) or TFD (Thin Film Diode) type which perform display in various display modes such as TN (Twisted Nematic) VA (Vertical Alignment), and IPS (In-Plane Switching) modes in an active-matrix mode using TFT or TFD elements and those of an STN (Super Twisted Nematic) type which perform display in an STN liquid crystal display mode in a passive-matrix mode.
The LCD devices of the TFT and TFD types are characterized by excellent picture quality and response speed and have been widely used in lap top computers, liquid crystal monitor, or liquid crystal television receivers. Although the STN type LCD devices are a little poor in picture quality and response speed, compared with those of the TFT type, they can be produced at a low cost due to their simple structures and low electricity consumption and can be used widely as displays for cellular phones, personal digital assistants, mobile personal computers, and mobile television receivers.
The above-described STN type liquid crystal display devices are generally structured so that a liquid crystal cell is sandwiched between a pair of polarizers and further provided with a reflector on the outside of either one of the polarizers when used in a reflection type liquid crystal display device.
The STN type liquid crystal display (may be referred to as “STN-LCD” hereinafter) device can not avoid an undesired coloration of displayed colors because optical rotating properties and birefringence are utilized on their displaying principle. In order to perform black and white display and further color display with overcoming the above-mentioned coloration, a method using an optical element for compensation has come into wide use. At the early stage, a method so-called double layer liquid crystal mode (D-STN: Double-layer STN) had been used wherein a compensation liquid crystal cell having a retardation (product of birefringence Δn and layer thickness d) substantially equivalent to that of a driving liquid crystal cell and having a twisted structure whose twist angle is the same as and twist direction is opposite to those of the driving cell is arranged for compensation such that the adjacent orientation axes of the driving- and compensation-liquid crystal cells form a right angle (Television Gakkai Technical Report by Osamu Okumura, 11, 79 (1987); Jpn. J. Appl. Phys., by K. Katou, 26, L17, 784 (1987); SID Digest by Y. Nagase, 1989).
However, although this method can achieve a high contrast display, it had been used for a short period of time because it had disadvantages that it increased the cost and the weight and thickness of the LCD device due to the use of two liquid crystal cells. Currently, a method has been used wherein pseudocompensation is performed with an optical retardation film obtained by precision-stretching a polymeric film such as polycarbonate. However, while the method using such an optical retardation film is simple and inexpensive, it has a problem that the undesired colored display of an STN-LCD device can not be substantially avoided due to the absence of a twisted molecular orientation in the optical retardation film.
As a third compensation method, there have been proposed methods using a liquid crystal film with a fixed twisted nematic liquid crystal orientation as an optical compensation film, as disclosed in Japanese Laid-Open Patent Publication Nos. 3-87720, 3-291620, 3-291623, 3-294821, and 4-003020. This optical compensation film has a twisted structure between molecules and thus is an excellent film having both an excellent compensation effect achieved by the method using a compensation liquid crystal cell and advantages of the method using an optical retardation film, i.e., easiness in use and low cost.
This compensation method is similar in principle to the D-STN method and optically compensates in the non-selective voltage application period. Therefore, since the polarization state of a light passing through the incident-side polarizer, the optical compensation film, and the driving liquid crystal cell becomes linearly polarized in all the wavelengths in the non-selective voltage application period, substantially perfect black display and substantially perfect white display can be obtained in the normally black mode (black in the non-selective voltage period) and in the normally white mode (white in the non-selective voltage period), respectively by properly selecting the position of the absorption axis of a polarizer of the output side.
On the other hand, in the selective voltage application period, the polarization state of a light passing through the incident-side polarizer, the optical compensation film, and the driving liquid crystal cell becomes elliptically polarized, unlike in the non-selective voltage application period. Furthermore, the elliptically polarized light varies in ellipticity and azimuth angle depending on wavelength. Therefore, since the selective voltage application period fails to provide perfect white or black display in the normally black mode or the normally white mode, unlike in the non-selective voltage application period, the development of more proper optical means has been required.
The object of the present invention is to provide a liquid crystal display device which is capable of providing a high-contrast white display with excellent brightness and hue in the normally black mode and a bright white-paper display and excellent black display in the normally white mode.